antidisestablishmentarianism - 28 letters
otorhinolaryngological - 22 letters
immunoelectrophoretically - 25 letters
psychophysicotherapeutics - 25 letters
thyroparathyroidectomized - 25 letters
pneumoencephalographically - 26 letters
radioimmunoelectrophoresis - 26 letters
psychoneuroendocrinological - 27 letters
hepaticocholangiogastrostomy - 28 letters
spectrophotofluorometrically - 28 letters
pseudopseudohypoparathyroidism - 30 letters